KISH > grassroots > chemistry
Each atom has a nucleus inside and electrons zooming around outside the nucleus. The size of an atom depends on how far away its outermost (valence) electrons are from the nucleus. If they are very close to the nucleus, the atom will be very small. If they are far away, the atom will be quite a bit larger. So the atomic size is determined by how much space the electrons take up.
There are comparisons in size in families and in a periods on the periodic table. In a family like from hydrogen to lithium to sodium on down--the atomic size increases because there are more protons but there are more orbits of electrons in the atom so it takes up more space with the extra electrons so as you go down a group, the size increases. As you go across a period, as from lithium to neon, the size decreases because there are more protons in the atoms but the electrons are are all in the same orbit so they are pulled closer to the nucleus so the size smaller. You need to remember those trends.
As you go right on the periodic table atomic size decreases
---------------> atomic size decreases
As you go down on the periodic table atomic size increases